[USRP-users] B2X0 crazy RSSI/Signal strenght problem

Marcus D. Leech mleech at ripnet.com
Tue Sep 9 09:06:49 EDT 2014

On 09/09/2014 05:21 AM, cFischbeck via USRP-users wrote:
> Hi,
> I have a strange problem with measuring the strengh of a signal.
> My first setup was a B2x0 with my custom firmware directly fed by a 
> Rhode&Schwarz SM300, (somewhere below 2.4GHz, no modulation, via cable).
> I've measured different signal levels, -30 to -120, and read out the 
> RSSI of the AD9361 and everything looked as intended. (B.t.w. is a 
> 20dB offset normal for that frontend?)
> Then i used another transmitter which has a DAT-31R5-SP+ as an 
> attenuator. The transmitter has a FM-modulator but the input is not 
> used (little bit of noise from surrounding electronics is still 
> modulating on a few kHz).
> The RSSI-Readouts where not as intended, maybe a 3 db change while the 
> configured change was 30dB. With Antennas on, the dopplereffect, where 
> the strenght changed as somebody moved in the room, was still there.
> First i though the transmitter isn't working but testet it with a 
> Rhode&Schwarz FSL 9kHz-3GHz Spectrum Analyser and the tx attenuation 
> works perfectly.
> So I used a B2X0 with normal firmware and HDSDR, connected with the 
> tx. Again the attenuation as no effect, while the the dopplereffect is 
> still there (with antennas). Then i used dedicated attenuators, which 
> goes inbetween the cable and the sma plug. That attenuator had the 
> expected drop of the spectrum in HDSDR.
> So the R&S FSL can measure signal strenght changes from the tx but the 
> B2x0 can't (no noticeable change in spectrum).
> The signal strenght changes throught the dedicated attenuators can be 
> measured by both.
> Have somebody an idea whats going on? Is this a mathematical problem 
> of the spectrum calculation?
> Have a nice day,
> Carsten
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How are you measuring RSSI?

The RSSI on front-end RF chips is generally not that useful because its 
observed bandwidth very rarely matches the bandwidth you're actually

How are you producing your spectrum?

If the signal strength isn't changing (much) despite the power levels 
being varied over 30dB, then either there's a problem in the way you're
   calculating RSSI, or the LNA on the B2xx has become damaged.

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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